This invention relates to a process for the preparation of sulfamic acid by reacting urea, sulfuric acid, and sulfuric anhydride, and more particularly, it relates to processes for preparing sulfamic acid with effective means for dealing with the heat released in such processes.
There are two main methods of preparing sulfamic acid, HSO.sub.3 NH.sub.2, namely, the direct reaction of anhydrous ammonia with sulfuric anhydride, and the reaction of urea with sulfuric acid and sulfuric anhydride.
In the first method, the reaction between SO.sub.3 and NH.sub.3 results, in a first stage, in the formation of ammonium sulfamino salts which have to be acidified and hydrolyzed with sulfuric acid in a second stage, thus leading to the formation of sulfamic acid and a greater or smaller quantity of ammonium hydrogen sulfate, known as ammonium bisulfate, which is separated by crystallization and filtration in the form of ammonium sulfate, after neutralization of the mother liquors of crystallization of the sulfamic acid. This type of process has the disadvantage of leading to the formation of at best one molecule of ammonium hydrogen sulfate per molecule of sulfamic acid formed, the under-production often being much greater in practice.
In the second method, the sulfamic acid is formed from sulfuric acid, sulfuric anhydride, and urea according to the general reaction scheme (I): EQU H.sub.2 SO.sub.4 +SO.sub.3 +CO(NH.sub.2).sub.2 .fwdarw.2 HSO.sub.3 NH.sub.2 +CO.sub.2 (I)
which has the theoretical advantage of not resulting in the formation of ammonium hydrogen sulfate.
In a first embodiment of this latter process, the urea is reacted with a substantial excess of sulfuric acid and anhydride, these two reactants being added either separately or in the form of mixtures of oleum and sulfuric acid. In this way, a suspension of sulfamic acid in a weak oleum or in sulfuric acid is obtained. To separate the sulfamic acid, it is necessary to perform filtration and washing of the cakes obtained with increasingly dilute sulfuric liquors. These filtering and washing operations are difficult to carry out and require a great deal of expensive apparatus. Moreover, the washing operations always cause partial hydrolysis of the sulfamic acid, leading to a loss of this product in the form of ammonium hydrogen sulfate.
In another process for the preparation of sulfamic acid from urea, in a first stage the sulfuric acid and urea are reacted in stoichiometric proportions with a quantity of sulfuric anhydride which may be in excess at a temperature below 50.degree. C., then in a second phase the liquid complex formed is decomposed at a temperature of 60.degree. C. to 100.degree. C., while the excess SO.sub.3 and the CO.sub.2 formed are entrained by means of an inert gas. It has been proposed to effect this decomposition in the presence of a third substance which may be, for example, recycled sulfamic acid in a quantity sufficient to ensure that the mass retains the appearance of a dry product, or an inert liquid compound immiscible with HSO.sub.3 NH.sub.2, such as tetrachloroethane, C.sub.2 H.sub.2 Cl.sub.4, or sulfuric chlorohydrin, HSO.sub.3 Cl. The decomposition is then carried out in a reactor with vigorous agitation, so as to break the sulfamic acid into small granules. One of the difficulties of this technique, which is otherwise useful, is the elimination of the heat released by the reaction (I) for forming the complex, since the very viscous liquid obtained has to be conveyed to a heat exchanger outside the reactor.